Research during the past year have focused on the lipids of the membranes of wild-type Acholeplasma laidlawii and comr-328, a mutant strain resistant to the lytic action of the terminal complex of complement proteins. Two important observations have been made: 1. When the wild-type strain is grown at 31 degrees C it is more resistant to complement-mediated killing than when grown at 37 degrees C. This confirms an earlier but unconfirmed observation from this laboratory. The resistance of the wild-type cells when they are grown at 31 degrees is in accord with our previous findings that the cells become resistant when grown at 37 degrees C in a fatty acid-free medium supplemented specifically with unsaturated rather than saturated fatty acids. It is also in accord with our observation that the resistant cells of the mutant strain, when grown in undefined medium, have a relatively high proportion of unsaturated fatty acids in their cell membranes, and they are resistant to the action of the terminal complex. In contrast, the wild-type cells grown under the same conditions, have a higher proportion of saturated fatty acids in their cell membranes, and they are sensitive to the lytic action of the proteins of the terminal complex. 2. We have undertaken a detailed quantitative analysis of the membrane lipids of the wild-type and mutant strains and have found the mutant strain is deficient in glycolipids. This observation allows us to focus on the role that these lipids play in the action of the terminal complement complex to the lytic action of the terminal complex. Our research program for the coming year will center on studies of the membrane lipid composition and response of wild-type and comr-328 strains when grown under difficient regimes of temperature and fatty acid supplements. We shall in addition, be studying the membranes of the wild-type and mutant strains by differential scanning calorimetry and 13C-NMR in order to gain an understanding of interaction between their membrane lipids and the proteins of the terminal complex.